Hematopoietic stem cells sustain the lifelong production of the vital blood lineages owing to their dual capacity for self-renewal and differentiation into all of the mature blood cell types. The choice between the maintenance of stem cell multipotency and differentiation is achieved through a complex interplay between intrinsic stem cell factors and extrinsic signals that originate from the surrounding microenvironment or niche. The complexity of the mammalian hematopoietic organ presents a significant challenge to the identification of the key molecular signaling pathways that regulate stem cell/niche interactions in vivo. One approach to this problem uses simple model systems to study the stem cell/niche system in the context of the whole organism. Several studies have identified a Drosophila hematopoietic multipotent progenitor with stem cell characteristics (hematopoietic stem-like cell). Stem-like cell potency is maintained by the niche through the Hedgehog, Wingless and JAK/STAT signaling pathways. Our studies demonstrate that the Friend of GATA factor U- shaped (Drosophila FOG; dFOG) is a key regulator of stem-like cell potency and that activated STAT directly upregulates dFOG gene expression. These findings position dFOG as a key link between the stem cell regulatory machinery and the stem cell niche. Loss of dFOG function is associated with increased cell proliferation and differentiation, a phenotype that is mimicked during immune challenge by wasp infection. We propose that dFOG functions as a key regulatory node that integrates multiple signals to control stem-like cell potency and differentiation during both normal hematopoiesis and in response to environmental stress. The goal of this application is to elucidate molecular mechanisms by which dFOG is regulated and to identify targets regulated by dFOG that control stem cell stem cell potency and differentiation in vivo. The specific hypotheses to be tested are that: 1) dFOG function must be repressed for stem-like cell differentiation to proceed in response to immune challenge; 2) dFOG integrates signals from multiple pathways to control stem- like cell potency and differentiation; 3) the GATA:FOG complex activates DE-cadherin gene expression to maintain stem-like cell potency. These hypotheses will be tested in the following specific aims: 1) Test the hypothesis that dFOG functions to repress the response of the stem-like cell to immune challenge; 2) Identify the regulatory inputs that control dFOG gene expression in stem-like cells; 3) Test the hypothesis that the GATA:FOG complex activates DE-cadherin gene expression to maintain multipotency. At the completion of these aims we anticipate that we will have identified key components of a regulatory cascade that controls stem cell potency in vivo. These studies are significant not only for our understanding of how hematopoietic stem cells are regulated in vivo, but are also critically important to realize the full potential of stem-cell-based regenerative therapies and to understand stem-cell-related disease.